Process for the removal of SO2 from gas streams

ABSTRACT

The removal of SO 2  from two or more gas streams of differing SO 2  content is accomplished by feeding each of the streams to an absorbent so that the more dilute SO 2  -containing stream or streams is supplied to the absorbent upstream from the point at which the more concentrated stream or streams is supplied. By supplying the gas streams to the absorbent flow in ascending order of SO 2  concentration, the amount of SO 2  absorbed by the absorbent flow is increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the removal of SO₂ from two or more gas streams.

2. Description of the Prior Art

It is known to remove SO₂ gas from a gas stream by absorbing the SO₂ in a suitable absorbent. Examples of some prior art patents showing such a technique are: U.S. Pat. Nos. 2,563,437 and 3,047,364.

In certain situations, two or more gas streams containing differing SO₂ levels will require treatment to remove the SO₂ from each stream. It has been commonplace to combine such gas streams into a single stream prior to contact with the absorbent. In order to recover the SO₂ values from such a resulting stream, large quantities of absorbent solution must be circulated due to the limitations of SO₂ equilibrium solubility. Hence, a need exists for a more efficient way to absorb SO₂ from two or more gas streams which contain varying SO₂ levels.

In U.S. Pat. No. 4,123,507 to R. H. Hass two SO₂ streams, presumably of differing SO₂ content, are not combined and are supplied to an absorber at two differing points. This patent, however, fails to indicate the degree of difference of the SO₂ content of the streams, fails to indicate which stream is the more concentrated, and fails to indicate any advantage for not combining the gas streams into a single stream prior to contact with the absorber. The patent illustrates recirculation of SO₂ -rich absorbent through the absorber which would tend to equalize the concentration of SO₂ gas in the absorber.

SUMMARY OF THE PRESENT INVENTION

The present invention is a process for the removal of SO₂ from two or more gas streams each having a differing SO₂ content. The process comprises providing a flow of liquid absorbent for the SO₂ -containing gas streams through at least one absorbent zone and supplying the gas streams to the absorbent flow in ascending order of SO₂ concentration so that the streams are supplied to the flow such that the more SO₂ dilute gas stream, or streams, is supplied to the flow upstream from the more SO₂ -concentrated gas stream, or streams, in order to increase the amount of SO₂ absorbed in the absorbent zone, or zones, by the absorbent flow.

In the present process, advantage is taken of the equilibrium solubility of SO₂ to minimize absorbent circulation and thus minimize capital and operating costs. By supplying the more concentrated SO₂ stream, or streams, to the absorbent flow without being diluted by the more dilute SO₂ stream while also contacting the more dilute SO₂ stream with the more lean absorbent, higher SO₂ loadings are achieved for the present process.

DESCRIPTION OF THE DRAWINGS

The Drawings, which form a part of the present specification, illustrate certain embodiments of the present invention wherein:

FIG. 1 is a schematic view of one embodiment of the present invention utilizing a single absorption column; and

FIG. 2 is another embodiment of the present invention showing use of a series of absorption columns.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention is illustrated in FIG. 1 wherein the process is used to remove SO₂ from a tail gas source containing arelatively low (e.g., less than 2 vol. %) SO₂ content and from a second, somewhat higher (e.g., over 2 vol. %) SO₂ -containing source from the incineration of excess H₂ S from a Claus reactor, e.g., an aqueous phase Claus reactor.

The tail gas source can be a Claus plant incinerator, steam boiler, or the like, with the SO₂ -containing gas being fed via line 10 to an intermediate portion of absorber 11. Lean absorbent solution is fed to thetop of the absorber via line 12 and can be any SO₂ absorbent known to persons of ordinary skill in the art. Exemplary absorbents include alkali metal phosphate buffered or unbuffered aqueous solutions, alkali metal citrate buffered or unbuffered aqueous solutions, and the like.

The second, more concentrated SO₂ source results from the incinerationof excess H₂ S from an aqueous regeneration/aqueous phase Claus reactor 13. This incinerated SO₂ is fed via line 14 to a point in thebottom portion of absorber 11 which is downstream from the lower concentration SO₂ stream fed via line 10.

The absorbent, which becomes enriched in SO₂, is removed from absorber11 by means of line 15 and is sent to the Claus reactor 13 where the SO₂ contained in the absorbent reacts with a supply of H₂ S forming sulfur in accordance with the classical Claus reaction carried outin an aqueous phase which is sent via line 16 to sulfur removal apparatus. The lean absorbent solution is then recycled to the absorber 11 by line 12.

FIG. 2 illustrates another embodiment of the present invention in which three streams of varying SO₂ content are each treated in three absorbers connected in series.

The stream 21 is lowest in SO₂ content (e.g., from the tail gas of a steam boiler) and is fed to absorber 22 with absorbent being supplied via line 23. The absorbent containing absorbed SO₂ is then fed via line 24 to second absorber 25. A second stream 26, somewhat more concentrated in SO₂ content (e.g., the incinerated tail gas from a Claus reactor),is fed to absorber 25 with the absorbent containing additionally absorbed SO₂ fed via line 27 to absorber 28. The highest SO₂ -containing stream (e.g., from incineration of excess H₂ S from an aqueous Claus reactor 29) is fed via line 30 to the third absorber 28 which is downstream from absorbers 22 and 25.

Absorbent effluent from absorber 28 is relatively rich in absorbed SO₂and is fed to absorbent/regeneration reactor 29 for combination with H₂ S. The resulting Claus reaction yields a liquid stream at 31 whichis passed to appropriate sulfur removal apparatus with the lean absorbent solution being recycled via lines 32 and 23 to the first absorber 22 for contact with the most dilute SO₂ -containing stream 21.

Depending upon the amount of SO₂ removal required, the gaseous effluents from each absorber can either be vented as shown in FIG. 2 or can be further treated by feeding them into the next absorber (e.g., the gaseous vent on absorber 28 can be fed to absorber 25 for further treatment or the gaseous vent on absorber 25 can be fed to absorber 22 forfurther treatment).

The process of the present invention, by insuring that the gas streams are supplied to the lean absorbent in ascending order of SO₂ concentrations, results in higher SO₂ equilibrium concentrations in the absorbent thereby allowing for a concomitant reduction in the absorbent solution circulation rate.

The process conditions used to absorb SO₂ in the present process are the conventional conditions well known to persons of ordinary skill in theart. Generally speaking, the absorption can take place at temperatures ranging from about 35° C. to about 80° C. with the pH of theabsorbent preferably in the range of from about 2 to 6.5. Similarly, the Claus reaction, and incineration and sulfur removal steps illustrated in connection with the present invention are in accordance with conventional practice.

The following Examples illustrate the equilibrium solubility characteristics of SO₂ under various conditions.

EXAMPLE 1

A series of experiments were conducted in absorbing SO₂ gas into a phosphate absorbent of the type described in U.S. Pat. No. 3,911,093 to F.G. Sherif et al. The loading values were obtained by circulating the absorbent solution at a fixed temperature and at a constant SO₂ gas inlet composition until the SO₂ gas content in and out of the absorbent zone was the same. The gas was saturated with water at the column operating temperature in order to maintain the water balance. Listed below were the results obtained at 74° C.±1° C. using an absorbent having the following characteristics: SO₄ : 57.5 gm/l.; 1.57M Na; 1.0M P; 0.26 gm/l. S_(x) O₆ ; pH: 4.4±0.2; and SO₂ O₃ : about 11.5 gm./l.

    ______________________________________                                                           SO.sub.2          pH at                                      Sample   SO.sub.2 in                                                                             Loading     S.sub.2 O.sub.3                                                                      Equi-                                      No.      Gas (%)  (gm/l.)     (gm/l.)                                                                              librium                                    ______________________________________                                         1        2.0      5.50        13.4  3.22                                       2        1.66     5.53        11.1  3.07                                       3        1.38     4.86        11.2  3.18                                       4        1.16     4.41        11.2  3.18                                       5        0.82     3.77        12.9  3.28                                       6        0.61     3.20        13.4  3.40                                       7        0.32     2.56        13.4  3.50                                       8        0.25     2.11        10.8  3.80                                       9        0.15     1.31        10.8  3.84                                       10       0.10     1.28        11.2  3.86                                       11       0.05     0.96        11.2  4.00                                       12       0.04     0.57        11.2  4.12                                       13       0.03     0.57        11.1  4.17                                       14       0.02     0.38        11.6  4.18                                       15       0.01     0.24        11.2  4.12                                       ______________________________________                                    

These data illustrate the general trend that the solubility of SO₂ in aqueous solutions is increased as the percent SO₂ in the inlet gas isincreased.

EXAMPLE 2

The data presented below illustrate the SO₂ loading values in the phosphate absorbent used in Example 1 for relatively dilute SO₂ -containing gas streams at two differing temperatures:

    ______________________________________                                                                      SO.sub.2                                          Sample   Temp.   SO.sub.2 in Loading                                                                               S.sub.2 O.sub.3                            No.      (°C.)                                                                           Gas (%)     (gm/l.)                                                                               (gm/l.)                                    ______________________________________                                         1        53      0.011       0.84   12.32                                      2        53      0.035       1.52   12.28                                      3        53      0.070       2.06   11.64                                      4        74      0.011       0.23   14.10                                      5        74      0.035       0.91   11.64                                      6        74      0.072       1.30   12.50                                      ______________________________________                                    

These data show that the SO₂ loading increases with increasing SO₂ content in the inlet gas and is also increased by use of lower temperatures.

EXAMPLE 3

The data presented below illustrate the SO₂ loading characteristics for gas streams containing a higher SO₂ content than the streams tested in Example 2:

    ______________________________________                                                                      SO.sub.2                                          Sample   Temp.   SO.sub.2 in Loading                                                                               SO.sub.2 O.sub.3                           No.      (°C.)                                                                           Gas (%)     (gm/l.)                                                                               (gm/l.)                                    ______________________________________                                         1        53      5.5         9.4    11.40                                      2        53      10.5        11.3   11.20                                      3        74      5.5         6.7    11.60                                      4        74      10.5        8.2    11.20                                      ______________________________________                                    

The same general trends noted for Example 2 also pertain.

EXAMPLE 4

Listed below are the SO₂ loading data using a somewhat higher temperature (i.e., 80° C.±1° C.) than used in any of Examples 1-3:

    ______________________________________                                                                SO.sub.2                                                          SO.sub.2 in  Loading  S.sub.2 O.sub.3                                Sample No.                                                                               Gas (%)      (gm/l.)  (gm/l.)                                        ______________________________________                                         1         1.64         5.03     11.60                                          2         1.12         4.84     11.82                                          3         0.62         3.77     11.82                                          4          0.225       2.32     12.28                                          ______________________________________                                    

The increased SO₂ content in the gas results in an increased SO₂ loading.

EXAMPLE 5

This shows the SO₂ loading values at 53° C.±1° C. forrelatively low PO₄ ⁻³ concentration absorbents (S₂ O₃ ⁻² : 11.7±0.4 gm./l. at P=1.0±0.2M and S₂ O₃ ⁻² : 6.2±0.1 gm./l. at P=0.53±0.1M):

    ______________________________________                                         % SO.sub.2 in Gas SO.sub.2 Loading (gm./l.)                                    P = 1.0  P = 0.53     P = 1.0  P = 0.53                                        ______________________________________                                         1.54     1.54         8.1      5.5                                             1.36     1.34         7.7      5.7                                             1.12     1.12         7.4      5.4                                             0.90     0.82         6.7      4.6                                             0.62     0.62         5.7      4.0                                              0.375    0.375       4.7      3.4                                             --        0.275       --       2.7                                              0.140    0.140       3.2      2.1                                             ______________________________________                                    

These data also illustrate the general trend that an increased SO₂ content in the inlet gas produces an increased SO₂ loading in the absorbent with a greater loading occurring as the phosphate concentration is increased for the relatively dilute PO₄ ⁻³ solutions used.

The foregoing Examples should not be construed in a limiting sense. The scope of protection desired is set forth in the claims which follow. 

What is claimed:
 1. A process for the removal of SO₂ from two or more gas streams each having a differing SO₂ content which comprises:(a) providing a flow of lean liquid absorbent for the SO₂ through at least one absorbent zone; and (b) supplying the gas streams containing the SO₂ to the absorbent flow so that the gas streams are supplied to the flow in ascending order of SO₂ concentration such that each more dilute SO₂ -containing stream is supplied to the flow upstream from each more concentrated SO₂ -containing stream in order to increase the amount of SO₂ absorbed by the absorbent flow.
 2. A process as claimed in claim 1 wherein a single absorber is used.
 3. A process as claimed in claim 1 wherein a plurality of absorbers is used, each connected in series to one another.
 4. A process as claimed in claim 3 wherein each absorber treats a single gas stream.
 5. A process as claimed in claim 1 wherein the most concentrated SO₂ gas stream is from the incineration of excess H₂ S from an aqueous phase Claus reactor.
 6. A process as claimed in claim 1 wherein the absorbent from the last absorbent zone is passed to an aqueous phase Claus reactor for reaction of the SO₂ contained therein with H₂ S.
 7. A process as claimed in claim 1 wherein the absorbent from the last absorbent zone is passed to an aqueous phase Claus reactor for reaction of the SO₂ contained therein with H₂ S with recycle of the resulting lean absorbent to the absorbent zone used to treat the most dilute SO₂ -containing gas stream.
 8. A process as claimed in claim 2 wherein absorbent from the last absorbent zone is passed to an aqueous phase Claus reactor for reaction of the SO₂ contained therein with H₂ S with recycle of the resulting lean absorbent to the absorbent zone used to treat the most dilute SO₂ -containing gas stream.
 9. A process as claimed in claim 3 wherein absorbent from the last absorbent zone is passed to an aqueous phase Claus reactor for reaction of the SO₂ contained therein with H₂ S with recycle of the resulting lean absorbent to the absorbent zone used to treat the most dilute SO₂ -containing gas stream.
 10. A process as claimed in claim 1 wherein a single absorber is used with the most concentrated SO₂ gas stream being from the incineration of excess H₂ S from a Claus reactor.
 11. A process as claimed in claim 10 wherein the absorbent from the last absorbent zone in the absorber is passed to an aqueous phase Claus reactor for reaction of the SO₂ contained therein with H₂ S.
 12. A process as claimed in claim 11 wherein lean absorbent from the Claus reactor is recycled to the absorbent zone used to treat the most dilute SO₂ -containing gas stream.
 13. A process as claimed in claim 1 wherein a plurality of absorbers is used, each connected in series to one another, with the most concentrated SO₂ gas stream being from the incineration of excess H₂ S from an aqueous phase Claus reactor.
 14. A process as claimed in claim 13 wherein absorbent from the last absorbent zone in the plurality of absorbers is passed to an aqueous phase Claus reactor for reaction of the SO₂ contained therein with H₂ S.
 15. A process as claimed in claim 14 wherein lean absorbent from the Claus reactor is recycled to the absorbent zone used to treat the most dilute SO₂ -containing gas stream.
 16. A process as claimed in claim 13 wherein each absorber treats a single gas stream.
 17. A process as claimed in claim 14 wherein each absorber treats a single gas stream.
 18. A process as claimed in claim 15 wherein each absorber treats a single gas stream. 